1. Field of the Invention
The present invention relates generally to the field of airships or buoyant vehicles and, more particularly, to a lightweight, lift producing, airframe and shell structure (“hull”) for a rigid body airship. A lift producing hull is highly advantageous because it allows an airship to take off in a statically negative condition, i.e., in a condition where the hull and payload weight exceed the maximum buoyancy supplied by the lifting gas.
2. Background of the Invention
Airships generate buoyant lift by displacing the surrounding air with a hull containing a lighter than air gas. Generally, there are three conventional types of airships, i.e., rigid, semi-rigid, and non-rigid designs. Most commonly, conventional airships have featured a cigar shaped hull, i.e., an elongated hull of circular cross-section. The conventional rigid airship uses a hull having an internal framework supported by multiple gas bags or cells. Similarly, the hull of a semi-rigid airship typically has a stiff internal lower keel for supporting a gondola underneath. A non-rigid airship on the other hand, has no rigid internal structure to support the hull. To maintain hull shape, semi-rigid and non-rigid airships rely upon lifting gas pressure, i.e., the skin of the hull of both types is primarily composed of a fabric material and the lifting gas has a small positive pressure which maintains the hull's shape. Rigid airships have a rigid external skin and therefore do not rely on lifting gas pressure to maintain structural integrity. At least one of each of the aforementioned conventional types of airship has been constructed and flown in the past.
Of the three conventional types of airships, i.e., fully rigid, semi-rigid and non-rigid, all have one salient feature in common, they depend solely upon the buoyancy generated by the lifting gas to create lift. These designs have numerous disadvantages. Conventional semi and non-rigid airships rely on internal pressure to maintain their form and therefore have limited forward airspeed because air pressure loads deform the nose of the hull. Conventional cigar-shaped rigid airships have the disadvantages of excessive weight due to the frame carrying all of the bending moments and aerodynamic loads. A cigar shaped hull is also unable to produce significant aerodynamic lift which further limits the payload capacity of the airship. Moreover, all conventional airships have the disadvantage of requiring ballast for payload exchange and thus require a ground handling crew to ballast the airship upon landing, further limiting utility.
In all types of airship, the hull may be regarded as a beam loaded by various forces which create moment and point loads in the hull. The first of the forces is buoyancy, where buoyancy is achieved by the lift created by the lighter than air lifting gas. Buoyancy introduces a distributed moment load over the hull. Buoyancy forces act upwardly or positively on the airship's hull. Air ship hulls are also affected by downwardly acting forces. These forces comprise the weight of the hull and any components attached to the hull. Of necessity, airships in addition to the weight of the hull include other downwardly acting weights such as engines, cockpits, fuel tanks, ballast, methods for compressing and storing the lifting gas and the like. Each of these weights introduces point loads into the hull and thereby creates a moment distribution across the surface of the hull. The moment distribution across the hull is further affected by the gas pressure bending moment. The gas pressure bending moment is an effect created by the tendency of helium to collect at the top of the hull, above the neutral axis of the hull which thereby introduces a differential in longitudinal loads (or a bending moment) through the hull.
In recent years, a new class of hybrid airships has been proposed. Hybrid airship designs feature a hull in the shape of a lifting body. These designs have the advantage of being able to generate aerodynamic lift in addition to the lift provided by the lifting gas. With the addition of aerodynamic lift, an airship can take off in a statically heavy condition and can thereby carry a heavier payload then would be possible with a conventional non-lifting body hull. Since substantial forward airspeed is need to generate aerodynamic lift, a rigid hull structure is needed to maintain hull shape under the stress imposed by aerodynamic loads. While rigid lifting body or hybrid airships have been proposed, to the inventors' knowledge, no such craft are presently operational. The reason for the lack of a successful lifting body airship is believed to be because aerodynamic lift introduces additional bending moments into the airship's hull which are substantially greater than those of conventional non-lifting body designs. These additional bending moments require a hull structure substantially stronger and hence heavier than that of a conventional rigid airship. Previously proposed rigid, lifting body airships have proposed hull designs incorporating conventional aircraft technology. These hulls however, if built, would likely prove so heavy as to render the designs nonfunctional much less practical for commercial or military use.
In view of the above, there is a need in the art for a hull design for a rigid, lifting body airship, that is light, simple, inexpensive to build and can readily withstand the loads imposed by aerodynamic lift.